r6040: use module_pci_driver macro

as the manual of module_pci_driver says that
it can be used when the init and exit functions of
the module does nothing but the pci_register_driver
and pci_unregister_driver.

use it for rdc's r6040 driver, as the init and exit
paths does as above, and also this reduces a little
amount of code.

Signed-off-by: Devendra Naga <devendra.aaru@gmail.com>
Acked-by: Florian Fainelli <florian@openwrt.org>
Signed-off-by: David S. Miller <davem@davemloft.net>